Seagrass landscape ecology
Eelgrass (Zostera marina) is the only native seagrass species in Washington state, where it forms narrow fringing beds and extensive meadow flats in soft-sediment areas and estuaries near mean lower low water. Much about the spatial structure of Pacific Northwest eelgrass communities remains unknown, including how diversity, abundance, and community structure differ between vegetated and unvegetated habitats, the direction and magnitude of edge effects, and the interrelated effects of bathymetry and patch size and shape. As a tech at the University of Washington in partnership with the state Department of Natural Resources, I studied how nekton (fishes and decapod crustaceans) respond to different aspects of eelgrass landscape structure, including patch size, vegetation presence/absence, and shoot morphology. This research was conducted on the traditional territories of the Chinook, Skokomish, S'klallam, Skagit, and Squaxin peoples.
In 2015, we found that nekton abundance increased from unvegetated habitat into eelgrass interiors in three sites across Washington, but that species richness was the same across habitats, differing only by site. Species and functional diversity showed the opposite pattern of abundance, decreasing into vegetated habitat. These results run counter to the commonly-accepted paradigm that the structurally complex habitat provided by seagrass increases diversity, richness and abundance relative to unvegetated habitat. We found that diversity declined in vegetated habitats because eelgrass disproportionately benefited only a few functional groups, such as pelagic schooling fishes and species that are cryptically-colored to match eelgrass blades.
|
In a study of nekton associations using capture (seine) and camera (GoPro) sampling, we found that edge habitats grouped with interior habitats in terms of nekton abundance and behavior, and that small pelagic fishes disproportionately benefited from seagrass structure. An exception occurred in videos, where abundance was greater on edges than interior or unvegetated habitats. These discrepancies were due to species-level variation in behavior and distribution patterns across habitats, which interact with sampling method biases to produce different observed patterns. For instance, pelagic schooling fishes moved more slowly in interior habitats, which could increase the likelihood of capture in seines in this habitat, but reduce the likelihood of appearing in videos.
|
In 2016, we compared abundance, diversity, and community structure across the eelgrass-unvegetated ecotone in fringes and flats. We found that abundance was elevated in vegetated habitat, but only in fringes, while abundance was the same across the ecotone in flats. Diversity and richness were greater in fringes than in flats too, across all habitats. We found no clear evidence of edge effects (which might be expected when comparing small narrow patches to extensive meadows), and there were no differences in small-scale shoot morphology between fringes and flats. These patterns may be caused by the steep gradient of depth and habitat types associated with fringes, which increase habitat heterogeneity over smaller spatial scales and provide refuges for small nekton species.
|